An FBI criminal investigation is underway after half a dozen inch-long sewing needles were found inside sandwiches served aboard Delta flights en route from Amsterdam to four different U.S. cities.

The news is alarming for travelers, especially as millions of Americans prepare to fly to London for the 2012 Olympic Games.

"Had I taken a big swallow and swallowed that down and had a big needle inside, that would be very concerning to me," said Dr. Jack Drogt, one of the airline passengers who found a needle in his food.

"I felt the poke in the top of my mouth," passenger James Tonjes recalled. "When pulled it out of my mouth it was very clear. I cleaned the food off and it was about a one inch long straight needle."

After two needles were found, Delta ordered all 18 of its flights from Amsterdam to stop serving sandwiches. One of the needles was actually found by a federal air marshal.

With the 2012 Olympic Games about to begin, airport security is tighter than ever. Consequently, this incident brings up new questions about the security of food on commercial planes.

"You can't check every sandwich that goes aboard a plane," former FBI agent Brad Garrett said. "Unfortunately, this demonstrates to people who want to do bad things that this is a gaping hole."

The FBI is trying to discover who had access to the food on the planes.

The sandwiches were prepared in the kitchen of Gate Gourmet, a catering company inside the Amsterdam airport. The company issued a statement saying it too is investigating the incident.

Meanwhile, as the investigation continues, Delta is serving only pre-packaged food. No other needles have been found.

"Delta is taking this matter extremely seriously and is cooperating with local and federal authorities who are investigating the incident," the airline said in a written statement.